Fix wasmer_api::query::fetch_webc_package

[theduke]

The current implemenetation has an issue:

Url::to_string() ends with a trailing slash, so the resulting would have double slashes.
We could just clone the URL and use Url::set_path().

But also, note that NamedPackageIdent has a registry field.

Originally posted by @theduke in #4532 (comment)